There are many processes and techniques for removing suspended particles from liquids. Traditionally, silica sand has been one of the major materials used for filtration purposes, both in the municipal and in the industrial sectors. However, a single-media filter, such as one of sand, has disadvantages which are well known to those in the filtration industry. A single-media filter bed is unable to handle high-solids loading without plugging. The need to handle higher-solids loading and, concurrently, to operate at higher filtration rates resulted in developing a dual-media filter composed of one or more layers of anthracite coal (specific gravity 1.48 to 1.65) over a layer of silica sand (specific gravity 2.65). Due to the differences between these two materials in specific gravity, the average particle size of the anthracite can be approximately 2 to 3 times that of the sand, and yet the anthracite will still remain above the sand when the filter is cleaned by back-flushing.
Since the development of dual-media beds, many attempts have been made to create a reverse-graded bed in which the particle size and, hence, porosity are largest at the surface where initial contact with suspended particles is made, gradually decrease throughout the bed depth, and are thus finest near the filter effluent. This reverse-graded bed has advantages over conventional dual-media beds. Attempts to create these reverse-graded beds have been made by using different materials which have different specific gravities. A number of patents have been issued for such beds, beginning with the Hyatt U.S. Pat. No. 293,745, and more recently with U.S. Pat. Nos. 3,382,983, 3,497,068 and 3,343,680. One attempt to create a reverse-gradation bed resulted in coating silica sand with a thermoplastic material having a specific gravity of less than 1. This is described in U.S. Pat. No. 3,704,786.
Additionally, many attempts have been made to manufacture synthetic filter media. Essentially, all of those utilize some form of thermoplastic material:
U.S. Pat. No. 3,544,457, Polyvinyl Chloride
U.S. Pat. No. 3,471,025, Polypropylene
U.S. Pat. No. 3,617,590, Thermoplastic
U.S. Pat. No. 3,557,955, Polyvinyl Chloride
U.S. Pat. No. 3,842,006, Thermoplastic
U.S. Pat. No. 3,502,434, Polyethylene
U.S. Pat. No. 3,463,320, Styrene
Unfortunately, all synthetic-plastic filter media cost 8 to 10 times more than conventional inorganic materials used for filtration. This has made their commercial utilization very limited.
Backflush filter beds for removing solids from liquids passed therethrough are reported in U.S. Pat. No. 3,343,680 to Rice, wherein plural layers of different filter media, e.g. nylon, different-density polyethylenes (Example 4), magnetite (Fe.sub.3 O.sub.4), ilmenite (FeO.TiO.sub.2), garnet, tabular alumina (heat-processed aluminum oxide), graphite rock, anthracite coal (widely used for water filtration) and silica sand (column 3, lines 1 to 22), are graded inversely with respect to size and specific gravity. As various filter media, such as anthracite coal, increase in cost and decrease in availability, substitute materials are sought.
As waste glass comprises from 8 to 12 percent by weight of municipal refuse and presents a severe disposal problem, both government and private agencies have devoted considerable energy and expertise to product development using such glass as raw material. In "Glasphalt", used for road surfacing, crushed glass is combined with asphalt. The U.S. Bureau of Mines has used waste glass for making such products as tiles, bricks and glass wool. Glass-polymer composite (GPC) materials, prepared from crushed waste glass, is useful for making sewer pipe. Foamed glass panels and other structural materials have been made from waste glass, and a number of patents, e.g. U.S. Pat. Nos. 3,811,851, 3,900,303 and 3,963,503, are directed to such foamed glass.
Spent sulfite liquor is another waste product which presents a disposal problem. It is a well-established and recognized product (cf. "The Condensed Chemical Dictionary", fourth edition, page 635, Reinhold Publishing Corporation, 1950; Clark, "The Encyclopedia of Chemistry", second edition, pages 1021 and 1022, Van Nostrand Reinhold Company, 1966; Grant, "Hackh's Chemical Dictionary", fourth edition, page 646, McGraw-Hill Book Company, 1969) which contains lignin or lignin sulfonate (also lignosulfonate) and is used as a tanning agent, a binding agent, a mordanting agent, an adhesive or for fermentation. The nature of lignin or of lignin sulfonate is confirmed by "The Condensed Chemical Dictionary", page 397, supra; "The Merck Index", ninth edition, page 717 (5326), Merck & Co., Inc., 1976; Grant, page 389, supra; and "McGraw-Hill Encyclopedia of Science and Technology", third edition, volume 14, pages 604 and 605, McGraw-Hill, Inc., 1971.
Lignins are commercially available in the form of a 45 percent by volume aqueous solution which is extremely viscous and difficult to work with. This solution is advantageously diluted to 9 percent by volume to provide a more easily usable product.
According to Freudenberg [Modern Methods of Plant Analysis, Vol. III (K. Paech and M. V. Tracey, eds.), page 507, Springer, Berlin, 1955; and Science, Vol. 148, page 595, 1965] lignin is considered to be a system of thermoplastic tridimensional polymers derived from coniferyl alcohol or other quaiacylpropane monomers. It is insoluble in water, in most organic solvents and in strong sulfuric acid. It contains the major portion of the methoxyl content of the wood. It has a characteristic ultraviolet absorption spectrum and gives characteristic color reactions with many phenols and aromatic amines. It has a variable elementary composition and methoxyl content. It reacts readily with sodium bisulfite or thioglycolic acid to form soluble products. It is unhydrolyzable with acids, readily oxidizes, is soluble in hot alkali, and readily condenses with alcoholic and phenolic compounds. It yields up to 25 percent vanillin when oxidized with alkaline nitrobenzene and Hibbert's monomers when boiled under reflux with ethanol and a little hydrogen chloride.